1. Field of the Invention
This invention relates to an antenna suitable for use in a radio terminal, and particularly to a slot antenna utilizing a coaxial cavity.
2. Description of the Related Art
A radio terminal has made progress in its size and thickness reductions in terms of an improvement in portability. At the same time, various types of small-sized antennas to be mounted to such a radio terminal have been developed. Of these, a slot antenna using a coaxial cavity has a feature that it can be built in the terminal without having to use a projection. In particular, a coaxial resonant slot antenna reduced in size by means of a structure wherein a center conductor (strip conductor) thereof is kept in noncontact with a cavity cubic (flat conductive cubic), has been proposed by Japanese Patent Laid-Open (Kokai) No. Hei 9-74312. An example of the structure thereof will be illustrated in FIG. 10.
The present antenna comprises a flat conductive cubic 1 shaped in a rectangular parallelepiped as a whole, a slender strip conductor 3, which extends along the direction of a resonant axis of a space inside the flat conductive cubic 1 and which is placed so as to be electrically insulated from the flat conductive cubic 1, and a radio wave transmitting and receiving slot 2 formed on the upper surface of the flat conductive cubic 1 so as to intersect the strip conductor 3. A RF source 7 feeds the RF power between a connection point 10 set to the strip conductor 3 and a side wall of the flat conductive cubic 1. The antenna can be much reduced to a few mm or less in thickness. Further, the antenna can be fabricated using the ordinary multilayer circuit board manufacturing process and has a great effect on size and cost reductions in terminal.
Since the antenna includes a cavity structure, the volume of the antenna is directly proportional to an impedance matching band. Thus, a problem arises in that when the antenna is applied to a terminal of a wide band radio communication system brought into large capacity using a plurality of carrier frequencies, an impedance matching frequency band to be ready for the antenna spreads and the volume of the antenna increases.
Since a phone call is normally made between a base station and a terminal, using one frequency shared between transmitting and receiving or two frequencies according to transmitting and receiving, a frequency band used at that time is very narrower than that of the entire system. Thus, if the impedance matching center frequency of the antenna can be adaptively varied to a frequency used on the phone for each phone call, then the frequency band to be possessed by the antenna can be reduced and hence the volume of the antenna can be lessened. Furthermore, a technique for adaptively varying the impedance matching center frequency of the antenna has been disclosed in Japanese Patent Laid-Open (Kokai) No. Shou 63-294107. However, the same publication contains no mention of the structure of the coaxial resonant slot antenna.